1. Field of the Invention
This invention relates to scatterometer systems and in particular to scatterometer systems which eliminate stray light. Further this system relates to scatterometers which in addition to having stray light eliminated are able to dump excess light without heating or other surplus light problems.
2. Description of the Prior Art
Previous systems used for scattering measurements from mirrors had no way of controlling back scattered light from entering the light emission source. The use of retroscatter in a device such as a laser gyro causes a problem of lock-in between the output wave from a laser and the reflected wave back along that path. When the waves lock together in frequency, no rotational information for a laser gyro can be obtained. The lock-in phenomena becomes a serious problem when two oscillators with only slight separation of operating frequencies couple together. In this case, there is a weak coupling mechanism which will cause both of them to oscillate at the same frequency. An example of this is the known fact that when two mechanical clocks have nearly the same frequency, they become weakly coupled acousticly by the sound each makes. The article "The Nonlinear Theory of Electric Oscillations" by Balth van der Pol in Proceedings of the Institute of Radio Engineers, Vol. 22, No. 9, September 1934, first analysed the lock-in phenomena as an effect in triode oscillators. A simplification of this analysis has since been published by Robert Adler in an article titled "A Study of Locking Phenomena in Oscillators" in the "Proceedings of the IRE and Waves in Electrons", Vol. 34, June 1946, page 351.
In the laser gyro, two oscillators are nominally independent with two counter propagating waves. These are weakly coupled by the light of one wave which retroscatters from the mirror and combines at a different phase and frequency with the light of the other counter propagating wave. When the frequencies of the two waves are sufficiently separated, which occurs when the laser gyro is rotating rapidly, the waves will not lock together. A recent paper on this laser gyro lock-in phenomena is "A Positive Scale Factor Correction in the Laser Gyro" by Fred Aronowitz and Wah L. Lim in the "IEEE Journal of Quantum Electronics," Vol. QE-13, No. 5, page 338, May 1977. At low gyro rotation rates, which imply small frequency separations, the waves lock together and no information about rotation rate can be obtained. There are two ways in which one can combat this lock-in phenomena. One is to apply a small mechanical oscillation motion called dither. This helps, but leads to a certain amount of undesirable random drift in the output. The second way to combat the lock-in is to develop lower scatter laser mirrors. To do this, more effective methods of measuring retroscatter are needed.